Effects of as-built surface with varying number of contour passes on high-cycle fatigue behavior of additively manufactured nickel alloy 718

Abstract (from the manuscript): High cycle fatigue life of laser-powder bed fusion (L-PBF) parts depends on several factors; as-built surfaces, when present, are a particular concern. This work measures as-built L-PBF surfaces with X-ray computed tomography, and uses rotating beam fatigue (RBF) testing to measure high cycle fatigue life. Surfaces with different, but consistent, characteristics are achieved by build in vertical specimens and changing only the number of contour passes. In this way, specimens with three different levels of surface roughness are compared to standard polished specimens. The results from this show that surface roughness increases with decreasing number of contour passes, but the impact on fatigue lifetime is not trivially related. Electron microscopy and x-ray computed tomography images show multiple initiation points and complex surface topology, but that surface feature depth is not necessarily correlated to failure location. Two primary conclusions are that 1) for the case of as-built surfaces loaded in bending, one contour pass is sufficient to achieve peak as-built performance, and even zero contour passes was not statistically significantly different from two passes; 2) surface feature depth, even when measured using metal-penetrating imaging, is not strongly associated with failure location in our data. When assessing AM surfaces, more factors (e.g., grain orientation) may be required to mechanistically understand the performance implications.